Micromechanical actuators are presently used in a variety of applications. For example, micromirrors are used in projector units which are designed for a very small installation space.
In such projector units, micromirrors which represent a so-called microelectromechanical system (MEMS) are used. Such MEMS mirrors frequently have multiple mechanical resonance points, also referred to as modes or poles in the transfer function, which may be appropriately electrically excited. In addition, such MEMS mirrors also have anti-resonance modes, also referred to as zero points in the transfer function, or notches.
The modes of the MEMS mirrors are subdivided into useful modes and spurious modes. In particular, the excitation of spurious modes adversely affects the quality of the projected image.
The mentioned MEMS mirrors form a so-called inert spring-mass system, which in a first approximation may be modeled as a second-order low pass (PT2 element). The cutoff frequencies of the inert spring-mass system are defined by the first mode thereof.
Such an MEMS mirror may be operated either resonantly in one or multiple useful modes, or quasi-statically. The quasi-static actuation is carried out using a low-frequency signal, and avoids excitation of the modes.
Image formation with the aid of MEMS mirrors usually requires two MEMS mirrors, one of the MEMS mirrors being actuated resonantly, and one of the MEMS mirrors being operated quasi-statically. The MEMS mirror operated resonantly is responsible for the line projection of the image, and the MEMS mirror operated quasi-statically is responsible for the line-by-line image formation.
An MEMS mirror is typically actuated using a sawtooth signal to generate a frame rate of 60 Hz, for example. In the frequency range, the sawtooth signal contains the multiples of the even and uneven harmonics of the fundamental frequency. Two possible sawtooth signals having different return times are illustrated in the diagram in FIG. 11 as dashed and solid curves. Time is plotted on the x-axis, and the amplitude of the sawtooth signal is plotted on the y-axis. FIG. 11 shows the rising edges of those edges which guide the MEMS mirror line by line. The falling edges represent the return of the MEMS mirror into its initial position. FIG. 12 illustrates the corresponding sawtooth signal in the frequency range.
Linear drivers or digital drivers are customarily used for actuating the MEMS mirrors in the quasi-static state. To achieve sufficient accuracy in the actuation or to increase the linear deflection, the micromirrors are actuated in a closed control loop. Various controllers, for example adaptive PD controllers, current controllers, and position controllers in a feed forward structure, LMS harmonic controllers, iterative harmonic coefficient determination, and the like may be used. The controllers which are used share the common feature that they require a very large system bandwidth, and therefore very high computing power.
A control system which operates according to the iterative harmonic coefficient determination method is provided in U.S. Pat. No. 7,952,783.
A large system bandwidth and a high computing power mean a large space requirement, for example for analog-digital converters, microcontrollers, digital-analog converters, driver stages, and the like, in the actuating ICs.
For example, systems having MEMS mirrors and controllers typically require a controller bandwidth of 1 MHz in order to accurately control each scan line. In addition, some of the known controller designs require additional pieces of status information concerning the MEMS mirror, which in reality are very difficult to detect or estimate.
In addition, modular multiple feedback controllers which have a simple design may be used for actuating an MEMS mirror.
Such modular multiple feedback controllers are less complex than other controller structures, and may therefore be implemented very easily, for example, in an ASIC or in software, for example as a program in a DSP or microcontroller.
The modular multiple feedback controllers for various MEMS mirrors are appropriately parameterized for achieving the best possible performance.